Broadly speaking, a treadle is a rubber strip over which vehicle tires roll and which is operative to record the passage of such tires. Typically such treadles are positioned within a rigid tray and buried in the upper surface of a roadway. When deformed, the treadle actuates an electrical counter for counting the number of vehicles passing over the roadway and/or the number of axles of individual vehicles.
For example, U.S. Pat. No. 4,455,456 discloses a treadle assembly that includes a plurality of elongated treadles of generally trapezoidal cross-section. The treadles are formed of a deformable, resilient material such as natural or synthetic rubber, polymer plastic or the like. These treadles are disposed within a steel tray in parallel extending, spaced apart relationship. Elongated retaining bars mounted on base plates are disposed between the treadles to retain the treadles within the tray.
The tray containing the treadles is then mounted on a support frame and buried within a roadway with the upper surface of the treadle extending slightly above the surface of the roadway whereby the tires of vehicles passing along the roadway will roll over the exposed treadles and deform them. Each treadle has a longitudinally extending hollow passageway therethrough containing a pair of elongated metallic, electrical contact strips attached to upper and lower surfaces of the passageway. As the tires of a vehicle cross a treadle, the treadle material is vertically deformed, causing the strips on the upper and lower surfaces of the passageway to be pressed together to close an electrical circuit and actuate a counter. The treadle assembly thus operates as an electrical switch to control a counter in response to a deformation force applied to the treadle by a vehicle tire.
Another compressible treadle switch assembly for use in a roadway is disclosed in U.S. Pat. No. 2,885,508. As in the previously mentioned patent, this treadle contains a longitudinally extending passageway having elongated, spaced apart electrical contact strips attached to upper and lower surfaces of the passageway. Vehicle tires rolling over the treadle deform the passageway to bring the strips into contact with one another to actuate an electrical counting circuit.
Treadles operating on the principle of electrical contacts suffer certain disadvantages. For example, care must be exercised in the shipping and handling of treadles that contain an electrical switch so as not to bend them and thus permanently deform and mechanically short circuit the contact strips. Because of this problem, it is necessary to securely pack such treadles in rigid crates for shipping purposes. Additionally, installing or replacing electrical contact treadles is expensive due to their design and fragility. The process typically requires two workers, an assortment of tools and three hours of roadway downtime. Further still, electrical contact treadles require about 60 pounds of pressure (psi) from a passing vehicle to operate, which generally necessitates limiting the speed of passing vehicles to 45 miles per hour (mph). This is problematic, however, because toll booths are increasingly using express lanes in which vehicles travel at higher rates of speed.
Another problem associated with treadles having electrical switches is that the presence of excessive moisture in the passageway can cause the electrical contact strips to short circuit. Because of this problem, it is essential that the passageway of such treadles be carefully sealed to prevent the introduction of moisture. Thus, the open ends of the passageways must be carefully sealed during treadle use.
Furthermore, treadles having electrical switches typically comprise two halves bonded together around the contact strips. Thus, extreme care must be taken during manufacturing to insure that no gaps exist between the halves such as would allow moisture to enter. However, because it is difficult to obtain an effective moisture seal in most cases, these treadles tend to permanently short out between the terminal strips after a relatively short period of use. On the average, such treadles must be replaced after only about six months of use due to excessive moisture in the passageway that has produced a permanent shorting of the contact strips.
More recently, U.S. Pat. No. 5,250,769 issued to the present inventor and incorporated herein by reference, disclosed an optical treadle assembly that overcomes the problems associated with electrical contact treadle assemblies. The treadle used therein is constructed of a deformable, resilient material that contains a hollow passageway throughout its length. A photocell emitter that generates a light beam and a photocell collector are located at opposite ends of the passageway. When the treadle is deformed by a vehicle tire, the passageway is at least partially closed and interrupts the light beam to the collector, by which the collector generates a signal indicative of the passage of the vehicle tire thereover. This signal may be used to operate a suitable counter or signal device as desired. The passageway is formed with an oval or elliptical cross-section such that the minimum diameter of the cross-section extends in a vertical direction to enhance closure of the passageway to interrupt the light beam when the treadle is compressed.
Because of the absence of electrical contacts, optical treadles are not damaged by the type of bending that occurs during ordinary handling. Thus, they may be allowed to bend and flex during shipping, handling and installation without causing damage thereto. Also because of the absence of electrical contacts, the switching apparatus in optical treadle assemblies is not susceptible to electrical short circuiting and permanent damage caused by the presence of excessive moisture in the treadle passageway. Thus, optical switching apparatus has a greater longevity than that of electrical contact switching apparatus.
Due to the longevity of the optical switching apparatus, the factor limiting life expectancy of an optical treadle assembly is the life expectancy of the treadle, which gradually becomes worn and/or permanently deformed during use. Once a treadle is worn or permanently deformed, the passageway may remain partially closed, continuously interrupting the light beam even when vehicles are not passing over the treadle. Thus no signal is generated that a vehicle tire has passed.
Thus, to minimize the replacement of treadles in the roadway assembly, there exists a need for a more durable treadle of increased longevity. To further increase treadle longevity, there exists a need for more reliable light beam interruption upon the passage of a vehicle tire. With such reliable interruption, the passage of a vehicle could be registered even with a worn treadle.
Additionally, because it is inevitable that even such an improved treadle will eventually fail due to wear, there exists a need for a roadway treadle assembly that allows a failed treadle to be quickly and easily replaced.